In this exercise, we will try to estimate sizes of sunspots on the image of the Sun. We will work out the sizes of other planets on this image.
The figure below shows an image of the sun projected on a sheet of paper. You have to quickly put cross ( X ) marks at the limb of the Sun as shown. Here we have put us just four marks for illustration. You may put more marks to get better accuracy. First, just put the cross marks as quickly as possible. You can number the marks later.
Once you have put the marks the paper can now be removed and is taken to the writing desk. Now number the cross marks as 1, 2, 3, … etc. and join two coss marks with a straight line.
Next, take perpendicular bisector of these lines. These are shown by red lines in the diagram below. You can see that the perpendicular meets at point C.
This point C is the centre of the image of the Sun. Measure distance C – X1, C-X2 etc and take an average of these numbers as the radius of the image. The diameter is twice this value.
You may repeat the exercise by joining point 3 and 1 or 4 and 2 and take the perpendicular bisectors again.
Trap: Though in principle all the perpendicular bisectors should meet at the same point, in practice this might not happen for various practical reasons (list the possible reasons).
Next, measure all the radius values and take the average as the radius of the projected image of the sun.
Diameter of the solar image will be twice the radius.
Here is our example.
The diameter of the image of the Sun = 96 mm
The diameter of the Sun = 13,92,000 km
1 mm = 1392000/ 96 = 14500 km
We can now measure the sizes of the sunspots and estimate their actual size on the solar disk.
The diameter of the Earth is 12742 km (from the web) and its size on this can be represented by a dot of diameter 0.9 mm (actually 0.879).
This might have now given you some feeling of how big the sun is and also most of the sunspots are much larger than the diameter of the Earth.
Trick: Make the image of the sun as large as you can to get better accuracy in measurements.
Next: note diameters of other planets from the internet, popular books or your text books. Calculate the scale down values for the image of the Sun and then draw the circle representing a planet.
DIYA 